The present disclosure relates to optical scanning devices and image forming apparatuses.
A typical image forming apparatus includes four photosensitive drums for four different colors to form a color image on a sheet. For simplifying the optical design and reducing the cost, such an image forming apparatus includes two laser scanning units (LSUs) for four photosensitive drums. That is, one LSU is shared between two photosensitive drums. Each LSU includes one polygon mirror and one motor. Each LSU rotates the polygon mirror by the motor and scans the two photosensitive drums with a laser beam, forming an electrostatic latent image on the respective photosensitive drums.
While being driven, the motors generate heat to raise the internal temperature of the housing of the LSUs, which may cause thermal deformation of the optical components or peripheral components. As a consequence, toner images of four colors, which are formed based on the four electrostatic latent images on the four photosensitive drums, may deviate from one another when the toner images are overlaid on a sheet. That is, misregistration of colors may occur.
In particular, when one polygon mirror is used to scan two photosensitive drums with light, the scanning direction on one of the photosensitive drums is opposite to the scanning direction on the other. Thus, images of the colors corresponding to the respective photosensitive drums deviate in opposite directions. Such deviation is more noticeable as compared with the case where color images are deviated in the same direction.
In view of the above, each LSU is provided with cooling passages, one below and another above the motor, for restricting the rise in the internal temperature of the housing. This is effective to restrict deviation of images of the respective colors. The cooling passage is located symmetrically relative to the rotation shaft of the motor.